In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. Pat. No. 5,819,704, the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. Specifically, it is well known that improved control of fuel metering, i.e. the rate of fuel flow into the combustion chamber, is essential in reducing the level of emissions generated by the diesel fuel combustion process while minimizing fuel consumption. As a result, many proposals have been made to provide metering, or injection rate, control devices in closed nozzle fuel injector systems. U.S. Pat. No. 5,779,149 to Hayes, Jr. discloses a piezoelectric controlled common rail injector of the servo-controlled type. The piezoelectric actuator controls the movement of an inwardly opening poppet-type control valve for controlling the flow of fuel from a control volume and ultimately the movement of the nozzle valve element. Fuel metering is variably controlled by controlling the duration and modulation of the electrical signal provided to the actuator. U.S. Pat. No. 5,713,326 to Huber discloses a similar injector design. Although these systems provide some control over fuel metering, nozzle valve opening and closing characteristics are sensitive to injection pressure, component tolerances and wear, fuel properties and temperature. Therefore, additional fuel metering control is desirable.
U.S. Pat. No. 5,860,597 to Tarr discloses a servo-controlled nozzle assembly for a fuel injector which operates to effectively control and vary the rate of fuel injection. The assembly includes a control valve element positioned in a control volume for cooperating with the needle valve element to control the drain flow of fuel through the drain circuit. Specifically, positioning of the control valve element relative to the valve surface controls drain flow through the drain circuit. A fast proportional actuator is used to permit selective controlled movement of the control valve element in proportion to the magnitude of the input signal to the actuator. However, this design does not offer any feedback information on actual valve lift which can be used for metering control. In addition, the control valve element engages a valve seat formed on the movable needle valve element and therefore may provide sufficient sealing in all situations as compared to a stationary valve seat.
Therefore, there is a need for a simple nozzle assembly for a fuel injector which is capable of effectively controlling fuel metering and providing variable control of the rate of fuel injection by sensing needle valve lift.